Optical discs provide a reliable long term storage medium available for digital information. In order to automate archived optical disc storage in a cost effective manner, optical discs are shelved for occasional automated retrieval. One method of storage is to place the optical discs in drawers where subgroups of optical discs can be accessed. However, an amount of empty space equal to or greater than the useable length of the drawer must be reserved for the drawer to slide open and the optical discs retrieved. In archival storage of optical discs it is desirable to minimize unused space in such storage configurations.
In addition to minimizing the amount of empty space needed for optical disc retrieval, it is also desired to minimize the amount of space that the optical discs themselves occupy so as to maximize optical disc storage density. As such, optical discs are stacked as closely together as possible. A problem with closely stacking optical discs is the subsequent process of accessing a specific optical disc within a tightly spaced optical disc stack. One technique of accessing a specific optical disc within an optical disc stack is to use a robotic gripper. The specific optical disc is accessed by moving the robotic gripper a determined distance within the optical disc stack based on an anticipated average thickness of each optical disc. However, variances in manufacturing tolerances for optical disc thicknesses results in inaccurate determination of a specific optical disc position within an optical disc stack.